An electrostatic charge is an electric charge which is stored on the surface of an insulated object. The process for charging the human body, for example, is known as triboelectrification--which occurs when an insulator and another material become charged by contact and separation, such as sliding. Shoe soles can become charged as the result of contact and separation with another insulating surface, such as carpet. The surface of clothes can become charged by contact and separation with seat coverings. Thus, a human body is initially uncharged. It is the presence of multiple charged sources located close to the body, such as footwear and clothing, that are responsible for the buildup of an electrostatic charge. The human body acts as a neutral conductor to transmit these charges, which are on the order of 1.times.10.sup.-6 C, to other bodies which may be either electrically grounded or at a floating potential. An ESD event occurs when a built up electrostatic charge is transmitted by one's touch, as the two bodies reach the same potential.
Electrostatic discharges, as described above, can damage and cause the failure of electronic components. For example, MOS devices are extremely susceptible to ESD damage because of their inherently high input impedances. Some submicron gate oxides can be ruptured by an electrostatic charge of between about 10-20 V, which can be easily built up via triboelectrification. Therefore, ESD protection of ICs is quite important.
Generally, electrostatic discharge protection is applied to the input/output (I/O) pins of an IC. One typical method of protecting an IC from ESD events is to connect each and every I/O pin to positive and negative supplies by reverse bias diodes. When an ESD event occurs, the diodes protect the IC from damage by providing low impedance discharge paths around sensitive internal circuitry of the IC for ESD pulses. However, when a transient electrostatic discharge occurs, these diodes may appear to the charge as either forward or reversed biased. As is well known, reverse bias diodes provide higher voltage discharge paths for an electrostatic discharge than forward biased diodes. In fact, power dissipated in a reverse bias diode due to an ESD event may be more than an order of magnitude greater than the power dissipated in the same diode when forward biased. Another unfortunate factor with reverse bias diodes is that they require larger junction areas than forward biased diodes. Large area protection diodes on I/O pins contribute unwanted parasitics, i.e., larger capacitances and leakage currents, which, in turn, degrade AC performance.
Clamping circuits have also been used in connection with electrostatic discharge paths through diodes. A supply clamp is typically connected across the power supply and limits the maximum differential voltage that can be applied between those two pins. Supply clamps are designed to act as voltage limiters by conducting large amounts of current when some predetermined maximum differential voltage, also known as a clamp voltage, is exceeded. A clamp provides a conduction path at a voltage higher than the power supply but lower than the breakdown voltage of the reverse biased I/O protection diodes, thereby reducing the reverse bias diode junction size requirement by reducing the power dissipation requirement.
While the addition of a clamp provides a greater level of electrostatic discharge protection, the clamp voltage is still set higher than the supply voltage. Therefore, the conduction voltage of all the ESD discharge paths through the supply clamp will be higher than the normal supply voltage. Accordingly, electrostatic discharge protection using I/O protection diodes with or without clamping cannot, in its standard form, provide voltage protection which is less than a normal supply voltage. For another example of an ESD protection device, see Ming-Dour Ker and Chung-Yu Wu, CMOS On-Chip Electrostatic Discharge Protection Circuit Using Four SCR Structures with Low ESD Trigger Voltage, Solid Stage Electronics, Vol. 37, No. 1, pp 17-26 (1994). Another ESD protector is disclosed in U.S. patent application Ser. No. 08/198,002, entitled "ESD Protection Using SCR Clamping" which was filed Feb. 17, 1994 and which is assigned to the assignee of the present invention. The '002 application describes an ESD protection device which may be clamped at a voltage level lower than a supply voltage. In this way, the invention can protect components which might otherwise be damaged by electrostatic discharges which are less than the voltage supply. However, the '002 invention disables some of its ESD protection when the IC is mounted on a printed circuit board. The invention also uses at least two additional dedicated pins on the IC.